130 



FORMATION OF MESOBLAST. 



end of the embryo, the mesoblast, the epiblast, and the hypoblast all 

 unite together ; just as they do in all the types of Ichthyopsida. 



Shortly afterwards, at a slightly later stage than that represented 

 in fig. 96, an important change takes place in the constitution of the 

 hypoblast in front of the primitive streak. The rounded cells, of which 

 it is at first composed (fig. 98), break up into (1) a layer formed of a 

 sino-le row of more or less flattened elements below — the hypoblast — 

 and (2) into a layer formed of several rows of stellate elements, between 

 the hypoblast and the epiblast— the mesoblast (fig. 99). A separation 

 between these two layers is at first hardly apparent, and before it has 

 become at all well marked, especially in the median line, an axial 

 opaque line makes its appearance in surface views, continued forwards 

 from the front end of the primitive streak, but stopping short at a 



Fig. 99. Transverse section through the embryonic region of the blasto- 

 derm or A CHICK shortly prior to the formation of the medullary groove and 

 notochord. 



m. median line of the section; ep. epiblast; l.l. lower layer cells (primitive hypoblast) 

 not yet completely differentiated into mesoblast and hypoblast ; n. nuclei of germinal 

 wall. 



semicircular fold — the future head-fold — near the front end of the 

 area pellucida. In section (fig. 100) this opaque line is seen to be 

 due to a special concentration of cells in the form of a cord. This 

 cord is the commencement of the notochord {ch). In some instances 

 the commencing notochord remains attached to the hypoblast, while 

 the mesoblast is laterally quite distinct {vide fig. 100), and is there- 

 fore formed in the same manner as in most Ichthyopsida ; while in 

 other instances, and always apparently in the Goose (Gasser, No. 127), 

 the notochord appears to become differentiated in the already sepa- 

 rated layer of mesoblast. In all cases the notochord and the hypoblast 

 heloiv it unite with the front end of the primitive streak ; with which 

 also the two lateral plates of mesoblast become continuous. 



From what has just been said it is clear that in the region of the 

 embryo the mesoblast originates as two lateral plates split off from 

 the hypoblast, and that the notochord originates as a median plate, 

 simultaneously with the mesoblast, with which it may sometimes be 

 at first continuous. 



Kolliker holds that the mesoblast of the region of the embryo is 

 derived from a forward growtK,|jj(3ii^he primitive streak. There is no 

 theoretical ol>jection tp^ffl|^?!^w\'ftn^^pt^kmk it would be impossible to 



